Kinetics and Mechanism of Oxidation of Hydroquinone by Tetrabutylammonium Tribromide Ion in Aqueous Acetic Acid

The oxidation of hydroquinone by environmentally benign tetrabutyl ammonium tribromide (TBATB) was carried out in 50% V/V aqueous acetic acid medium under pseudo-first-order conditions, keeping a large excess of hydroquinone over the oxidant. The main reactive species of oxidant and substrate were found to be the Br₃^-\mathrm{Br}_{3}^{-} ion and hydroquinone, respectively. The reaction proceeds with prior complex formation between the reactants followed by its slow decomposition to generate semiquinone and bromine radicals. The complex formation was kinetically verified by its Michaelis–Menten plot. The solvent effect was verified by using Grunwald–Winstein equation which is consistent with an S_N2 mechanism. The formation constants for the complex and rate constant for the slow decomposition step were determined by studying the reaction at five different temperatures. The values of formation constant of the complex and the rate constant for its decomposition were determined at these temperatures. The activation parameters with respect to the slow step of the reaction have also been determined.     

Kinetic studies on the bromate ion oxidation of 12-tungstocobaltate(II) in aqueous acid

The oxidation of Co^IIW by bromine(V) is a complex process involving an induction period. The reaction was found to be first-order in both [Co^IIW] and [Br^v], and exhibits a complex dependence on [H⁺]. These observations were successfully explained by considering HBrO₂, one of the intermediates formed in the direct but slow reaction between Co^IIW and bromine(V), as the reacting species. The first-order limiting dependence in [H⁺] was due to the involvement of a protic equilibrium of HBrO₂. The induction period appears due to the scavenging effect of Br^– inadvertently present in the medium. It appears to be the first report where HBrO₂ was found to be the reacting intermediate in the oxidation of metal ions and complexes by BrO^– ₃.     

Evidence of protonation during the oxidation of some aryl alcohols by permanganate in perchloric acid medium and mechanism of the oxidation processes

The oxidation of benzyl alcohol and methoxy-, chloro-, and nitro- substituted benzyl alcohols by permanganate has been studied in aqueous and acetic acid medium in presence of perchloric acid. The reaction is first-order in [MnO₄^?] and [XC₆H₄CH₂OH], but the order is complex with respect to [H⁺]. Different thermodynamic parameters have been evaluated. The reaction occurs through the protonation of alcohol in a fast preequilibrium followed by a slow rate-determining oxidation step. A two-electron transfer oxidation step has been suggested for benzyl alcohol and chloro- and nitro- substituted alcohols, while the oxidation of methoxy compounds involves a one-electron transfer via a free-radical mechanism. © 1995 John Wiley & Sons, Inc.     

On the Oxidation of the Three‐Dimensional Aromatics [B12X12]2− (X=F,Cl, Br,I)

The perhalogenated closo‐dodecaborate dianions [B₁₂X₁₂]^2? (X=H, F, Cl, Br, I) are three‐dimensional counterparts to the two‐dimensional aromatics C₆X₆ (X=H, F, Cl, Br, I). Whereas oxidation of the parent compounds [B₁₂H₁₂]^2? and benzene does not lead to isolable radicals, the perhalogenated analogues can be oxidized by chemical or electrochemical methods to give stable radicals. The chemical oxidation of the closo‐dodecaborate dianions [B₁₂X₁₂]^2? with the strong oxidizer AsF₅ in liquid sulfur dioxide (lSO₂) yielded the corresponding radical anions [B₁₂X₁₂] ^{? ?} (X=F, Cl, Br). The presence of radical ions was proven by EPR and UV/Vis spectroscopy and supported by quantum chemical calculations. Use of an excess amount of the oxidizing agent allowed the synthesis of the neutral perhalogenated hypercloso‐boranes B₁₂X₁₂ (X=Cl, Br). These compounds were characterized by single‐crystal X‐ray diffraction of dark blue B₁₂Cl₁₂ and [Na(SO₂)₆][B₁₂Br₁₂] ? B₁₂Br₁₂. Sublimation of the crude reaction products that contained B₁₂X₁₂ (X=Cl, Br) resulted in pure dark blue B₁₂Cl₁₂ or decomposition to red B₉Br₉, respectively. The energetics of the oxidation processes in the gas phase were calculated by DFT methods at the PBE0/def2‐TZVPP level of theory. They revealed the trend of increasing ionization potentials of the [B₁₂X₁₂]^2? dianions by going from fluorine to bromine as halogen substituent. The oxidation of all [B₁₂X₁₂]^2? dianions was also studied in the gas phase by mass spectrometry in an ion trap. The electrochemical oxidation of the closo‐dodecaborate dianions [B₁₂X₁₂]^2? (X=F, Cl, Br, I) by cyclic and Osteryoung square‐wave voltammetry in liquid sulfur dioxide or acetonitrile showed very good agreement with quantum chemical calculations in the gas phase. For [B₁₂X₁₂]^2? (X=F, Cl, Br) the first and second oxidation processes are detected. Whereas the first process is quasi‐reversible (with oxidation potentials in the range between +1.68 and +2.29 V (lSO₂, versus ferrocene/ferrocenium (Fc^0/+))), the second process is irreversible (with oxidation potentials ranging from +2.63 to +2.71 V (lSO₂, versus Fc^0/+)). [B₁₂I₁₂]^2? showed a complex oxidation behavior in cyclic voltammetry experiments, presumably owing to decomposition of the cluster anion under release of iodide, which also explains the failure to isolate the respective radical by chemical oxidation.     

Biphasic oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) by N-bromosuccinimide and the formation of chromium(IV) and chromium(V)

The kinetics of oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) complex, [Cr^III(phen)(H₂O)₂Cl₂]⁺, by N-bromosuccinimide (NBS) is biphasic. The first faster step involves the oxidation of Cr(III) to Cr(IV). The second slower step is due to the oxidation of Cr(IV) to Cr(V). The reaction product is isolated and characterized by electron spin resonance (ESR), IR, and elemental analysis. The chromium(V) product is consistent with the formula [Cr^V(phen)Cl₂(O)]Br. The rate constants k_f and k_s, for the faster and the slower steps respectively, were obtained using an Origin 9.0 software program. Values of both k_f and k_s, varied linearly with [NBS] at constant reaction conditions. The effect of pH on the reaction rate is investigated over the pH (4.11–6.01) range at 25.0°C. The rate constants k_f and k_s increased with increasing pH. This is consistent with hydroxo forms of the chromium species being more reactive than the aqua forms. Chromium(III) complexes, more often than not, are inert. The oxidation of the Cr(III) complex to Cr(IV), most likely, proceeds by an outer sphere mechanism. Since chromium(IV) is labile the mechanism of its oxidation to chromium(V) is not certain.     

Kinetics and mechanism of Ru(III)-catalyzed oxidation of aliphatic alcohols by trichloroisocyanuric acid in HClO4 medium

The kinetics and mechanism of Ru(III)-catalyzed oxidation of some aliphatic alcohols by trichloroisocyanuric acid (TCICA) has been studied in aqueous HOAc-HClO₄ medium. The reaction is zero order in [TCICA], fractional order in [alcohol] and first order in [Ru(III)]. The reaction is insensitive towards changes in acid concentration. The rate is not affected by an increase in [Cl⁻]. The polar reaction constant (ρ*) was found to be −1.27 at 308 K. A mechanism involving complex formation between the substrate and catalyst in the fast equilibrium step followed by its decomposition in a slow step is proposed.     

Kinetics and mechanism of oxidation of allyl,crotyl and cinnamyl alcohol by chromium(V)

Summary The kinetics of oxidation of unsaturated alcoholsviz. allyl, crotyl and cinnamyl alcohol by sodium bis[2-ethyl-2-hydroxy butanoato (2–)] oxochromate(V) Cr^v, has been investigated in 25% (v/v) aq. HOAc:HClO₄. The order in [oxidant] and [substrate] was 1.0 and 0.7 respectively. The oxidation rate increased with increase in [2-ethyl-2-hydroxybutyric acid] (EHBA) and decreased with increase in the percentage of HOAc. The rate decreases slightly with increase in [H^_]. The unsaturated alcohols exhibited higher reactivity compared to their saturated analogues. A mechanism involving the formation of a complex between Cr^v and alcohol which in turn disproportionates into products in a slow step is advanced to explain the kinetic results.     

Oxidation of alcohols by N-chlorosuccinimide—a kinetic study

The kinetics of oxidation of alcohols by N-chlorosuccinimide (NCS) have been studied. The independence of rate on the concentration and structure of alcohol and the fractional order dependence on concentrations of added H⁺ and Cl^? suggest that the reaction proceeds through the formation of Cl₂ generated in a steady concentration in a slow step followed by a rapid uptake of the alcohol. A rate expression for the observed kinetics has been suggested. This behaviour is contrasted with that of an analogous system viz, NBS and alcohol and is also compared with the Orton rearrangement.     

Ruthenium(III)‐catalyzed oxidation of cyclopentanol and cyclohexanol by N‐bromoacetamide

Kinetic investigations on Ru(III)‐catalyzed oxidation of cyclopentanol and cyclohexanol by acidic solution of N‐bromoacetamide (NBA) in the presence of mercury(II) acetate as a scavenger have been carried out in the temperature range of 30–45°C. Similar kinetics was followed by both the cyclic alcohols. First‐order kinetics in the lower concentration range of NBA was observed to tend to zero order at its higher concentrations. The reaction exhibits a zero‐order rate dependence with respect to each cyclic alcohol, while it is first order in Ru^III. Increase in [H⁺] and [Cl^?] showed positive effect, while successive addition of acetamide exhibited negative effect on the reaction rate. Insignificant effect of sodium perchlorate, D₂O, and mercury(II) acetate on the reaction velocity was observed. Cationic bromine has been proposed as the real oxidizing species. Various thermodynamic parameters have been computed. A suitable mechanism in agreement with the kinetic observations has been proposed. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 275–281, 2005     

Kinetics and mechanism of oxidation of anisole and substituted anisoles by acid bromate in acetic acid-water mixture

Oxidation of anisoles by acid bromate has been studied in acetic acid-water system in the presence of sulphuric acid. The reaction is first order each in [anisole] and [Br(V)]. The rate of reaction increased with increase in [H⁺] and percentage of acetic acid. The products of oxidation have been identified as ortho and para hydroxyanisoles. From the effect of [H⁺] and [acetic acid] on rate, H ₂ ⁺ BrO₃ has been established as the reactive species. Anisoles having electron-donating substituents in the benzene ring accelerate the rates and vice versa with a Hammett ρ value of −0.6. A mechanism involving the attack of H ₂ ⁺ BrO₃ on ortho/para position of the anisole in the rate-determining step has been proposed.     

Acylation Reactions of Dibenzo-7-phosphanorbornadiene: DFT Mechanistic Insights

Extensive DFT calculations provide deep mechanistic insights into the acylation reactions of tert-butyl dibenzo-7-phosphanobornadiene with PhCOX (X=Cl, Br, I, OTf) in CH₂Cl₂ solution. Such reactions are initialized by the nucleophilic P⋅⋅⋅C attack to the carbonyl group to form the acylphosphonium intermediate A⁺ together with X⁻ anion, followed either by nucleophilic X⁻⋅⋅⋅P attack (X=Cl, Br, and I) toward A⁺ to eliminate anthracene or by slow rearrangement or decomposition of A⁺ (X=OTf). In contrast to the first case (X=Cl) that is rate-limited by the initial P⋅⋅⋅C attack, other reactions are rate-limited by the second X⁻⋅⋅⋅P attack for X=Br and I and even thermodynamically prevented for X=OTf, leading to isolable phosphonium salts. The rearrangement of phosphonium A⁺ is initiated by a P-C bond cleavage, followed either by sequential proton-shifts to form anthracenyl acylphosphonium or by deprotonation with additional base Et₃N to form neutral anthracenyl acylphosphine. Our DFT results strongly support the separated acylphosphonium A⁺ as the key reaction intermediate that may be useful for the transfer of acylphosphenium in general.     

One-step three-electron oxidation of tartaric and glyoxylic acids by chromium(VI) in the absence and presence of manganese(II)

Kinetics of the initial stages of oxidation of tartaric acid (TA) in the absence and presence of manganese(II) ions have been studied spectrophotometrically. The rate of the induction was slow and then gradually increased with increasing [TA] and/or [HClO₄]. The reaction followed third-order kinetics; first-order with respect to each of [TA], [HClO₄] and [Cr^VI]. The kinetic and manganese(II) effect studies are consistent with a one-step three-electron mechanism (Cr^VI Cr^III without passing through Cr^IV as an intermediate) in which a termolecular complex is formed between TA, Mn^II and HCrO₄ ^–. In order to obtain further insight, oxidation of glyoxylic acid (GA), an oxidation product of TA, was also studied under the similar conditions. Details of the process are discussed.     

Functionalization of one or two methyl groups in the [Cp*<Subscript>2</Subscript>RuBr]<Superscript>+</Superscript>Br<Superscript>−</Superscript> complex in the reaction with bromine

The reaction of [Cp*₂RuBr]⁺Br⁻ with bromine in CH₂Cl₂ (CD₂Cl₂) in an inert atmosphere at room temperature produces the complexes [Cp*Ru(Br)C₅Me₄CH₂Br]⁺Br₃ ⁻ (syn conformer), [Cp*Ru(Br)C₅Me₃(CH₂Br)₂]⁺ (syn and anti conformers), and [Ru(Br)(C₅Me₄CH₂Br)₂]⁺ (syn conformer). All complexes were characterized by ¹H and ¹³C NMR spectroscopy; the former complex, by elemental analysis. These complexes were also prepared by the reaction of [Cp*RuC₅Me₄CH₂]⁺BF₄ ⁻ with bromine in CH₂Cl₂. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2712–2718, December, 2005.     

Complex dynamical behavior in the oxidation of hydroxylamine by bromate

Complex dynamical behavior has been observed in the oxidation of hydroxylamine by bromate in acidic sulfate medium. The reaction shows clock type kinetics in closed conditions and an aperiodic oscillations if gaseous products are removed from the system with a constant flow-rate. The reduction kinetics of bromate ions with excess hydroxylamine has been studied in the presence of allyl alcohol. The observed pseudo-first-order rate constant k_obs has been found to follow the expression where [hydroxylamine] is total initial hydroxylamine concentration, K₁ = 0.5 M^?1, K₂ = 10⁶ M^?1, and k = 2.57 × 10³ M^?1 s^?1 at 298.15 K and I = 2.0 M. The rate constant for the bromine oxidation of hydroxylamine in sulfuric aqueous solution has been determined. © 1994 John Wiley & Sons, Inc.     

Mechanistic study of cerium(IV) oxidation of antimony(III) in aqueous sulphuric acid in the presence of micro amounts of manganese(II) by stopped flow technique

The oxidation of antimony(III) by cerium(IV) has been studied spectrometrically (stopped flow technique) in aqueous sulphuric acid medium. A minute amount of manganese(II) (10⁻⁵ mol dm⁻³) is sufficient to enhance the slow reaction between antimony(III) and cerium(IV). The stoichiometry is 1:2, i.e. one mole of antimony(III) requires two moles of cerium(IV). The reaction is first order in both cerium(IV) and manganese(II) concentrations. The order with respect to antimony(III) concentration is less than unity (ca 0.3). Increase in sulphuric acid concentration decreases the reaction rate. The added sulphate and bisulphate decreases the rate of reaction. The added products cerium(III) and antimony(V) did not have any significant effect on the reaction rate. The active species of oxidant, substrate and catalyst are Ce(SO₄)₂, [Sb(OH)(HSO₄)]⁺ and [Mn(H₂O)₄]²⁺, respectively. The activation parameters were determined with respect to the slow step. Possible mechanisms are proposed and reaction constants involved have been determined.     

Biphasic Oxidation of cis‐Diaquabis(1,10‐phenanthroline)chromium(III) by N‐Bromosuccinimide and the Formation of Chromium(IV) and Chromium(V)

The oxidation of cis‐diaquabis(1,10‐phenanthroline)chromium(III) [cis‐Cr^III(phen)₂(H₂O)₂]³⁺ by ‐bromosuccinimide (NBS) to yield cis‐dioxobis(1,10‐phenanthroline)chromium(V) has been studied spectrophotometrically in the pH 1.57–3.56 and 5.68–6.68 ranges at 25.0°C. The reaction displayed biphasic kinetics at pH < 4.0 and a simple first order at the pH > 5.0. In the low pH range, the reaction proceeds by two successive steps; the first faster step corresponds to the oxidation of Cr(III) to Cr(IV), and the second slower one corresponds to the oxidation of Cr(IV) to Cr(V), the final product of the reaction. The formation of both Cr(IV) and Cr(V) has been detected by electron spin resonance (ESR). The ESR clearly showed the formation and decay of Cr(IV) as well as the formation of Cr(V). Each oxidation process exhibited a first‐order dependence on the initial [Cr(III)]. The pseudo–first‐order rate constants k₃₄ and k₄₅, for the faster and slower steps, respectively, were obtained by a computer program using Origin7.0. Both rate constants showed first‐order dependence on [NBS] and increased with increasing pH.     

Regio- and stereoselective addition of methane- and bromomethanesulfonyl bromides to 1-phenylthiotricyclo[4.1.0.0<Superscript>2,7</Superscript>]heptane

1-Phenylthiotricyclo[4.1.0.0^2,7]heptane reacted with MeSO₂Br and BrCH₂SO₂Br directly at mixing at 20°C in CH₂Cl₂ along a ionic (electrophilic with respect to bromine) mechanism affording a product of an antistereoselective addition to the central bicyclobutane C¹–C⁷ bond of the norpinane structure. The reaction product contains the exo-oriented sulfonyl group in the geminal position to the SPh substituent. The structure of the adduct with MeSO₂Br in a single crystal was determined by XRD analysis.     

Decomposition of [NiRR′L2] complexes induced by bromine or anodic oxidation

A study has been made of the decomposition of the compounds t-[NiRR′L₂] (L = PMe₂Ph and PEt₃; R = aryl or vinyl groups) and [Ni(mes)(o-tol)bipy] (mes = mesityl) oxidatively induced either by electrochemical means or by bromine. No organometallic compound of Ni^III was isolated in the above reactions, but a pentacoordinate intermediate of Ni^III is postulated. Breakdown takes place readily after the Ni^III intermediate is formed. If the decomposition is induced electrochemically, the intermediate decomposes giving only the coupling product R-R′. When bromine is used as the oxidizing agent, the Ni^III intermediate is only formed if coordination to the central atom is allowed by the volume of the ligands. Thus, [Ni(C₂Cl₃)(mes)(PMe₂Ph)₂] does not decompose at all, and only [Ni(C₂Cl₃)(mesBr₂)(PMe₂Ph)₂] is obtained. The intermediate “Ni^IIIRR′BrL₂” undergoes reductive elimination to give R-R′, RBr and R′Br. The formation of the products R-R′ is increasingly favoured the greater the electronegativity of the organic ligands. The reductive elimination giving RBr takes place more readily the greater the electronegativity of the organic ligand R. The product of the reductive elimination reaction is “Ni^IBr”, “Ni^IR”, or “Ni^IR′”, which in the presence of bromine give Ni²⁺, [NiBr(RBr)L₂], phosphonium salts, RBr, and R′Br.     

Kinetic study of 2‐propanol and benzyl alcohol oxidation by alkaline hexacyanoferrate(III) catalyzed by a terpyridyl ruthenium complex

The complex (Trpy)RuCl₃ (Trpy = 2,2′:6′,2″‐terpyridine) reacts with alkaline hexacyanoferrate(III) to form a terpyridyl ruthenium(IV)‐oxo complex that catalyzes the oxidation of 2‐propanol and benzyl alcohol by alkaline hexacyanoferrate(III). The reaction kinetics of this catalytic oxidation have been studied photometrically. The reaction rate shows a first‐order dependence on [RU(IV)], a zero‐order dependence on [hexacyanoferrate(III)], a fractional order in [substrate], and a fractional inverse order in [HO⁻]. The kinetic data suggest a reaction mechanism in which the catalytic species and its protonated form oxidize the uncoordinated alcohol in parallel slow steps. Isotope effects, substituent effects, and product studies suggest that both species oxidize alcohol through similar pericyclic processes. The reduced catalytic intermediates react rapidly with hexacyanoferrate(III) and hydroxide to reform the unprotonated catalytic species. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 760–770, 2000     

Efficient and Reusable Sn(II)‐containing Imidazolium‐based Ionic Liquid as a Catalyst for the Oxidation of Benzyl Alcohol

A simple and efficient catalytic system [BBIM]Br–SnCl₂ for the oxidation of benzyl alcohol using hydrogen peroxide as the oxidant has been developed. Reaction conditions such as the catalyst dose, the solvents, reaction temperature, reaction time, and the amount of hydrogen peroxide were investigated. The optimum reaction conditions identified were 0.11 g of catalyst, no solvent, 65°C, 15 min, and 2 mmol of hydrogen peroxide. Oxidation of various alcohols was also investigated under the optimized conditions. The catalyst [BBIM]Br–SnCl₂ can be easily recovered and reused for six reaction runs without significant loss of catalytic activity, because the Sn species of the catalyst can be coordinated with the imidazole ring of the ionic liquid. The reused catalyst was further characterized by Fourier transform infrared spectroscopy to evaluate its chemical properties. The results proved that the [BBIM]Br–SnCl₂ catalyst was stable and reusable for the oxidation reactions. A possible mechanism for the oxidation of benzyl alcohol to benzaldehyde is proposed.